Local electronic properties induced at the molecule-metal interface

Electron emission spectra resulting from thermal collisions of He*(2(3)S) metastable atoms with alkanethiols (CnH2n+1SH, n = 1-3) on a Pt(111) surface were measured to clarify the electronic properties induced at the molecule-metal interface. Upon chemisorption, the S 3p-derived states are broadened significantly by admixture with the Pt bands, yielding a sharp Fermi edge, whereas such a metallic state is missing in weakly bound species. The spectral feature is in agreement with first-principles calculations using density functional theory. Furthermore, our data show that the local density of states at the Fermi level decreases rapidly on going from the S atom to the outer C atoms, indicating that the chemisorbed species are metallic in character at the S end and insulating at the alkyl end. A new classification of adsorbates into insulating, metallic, and amphibious molecules would be useful for understanding ordinary charge transport at the molecule-metal interface.